If you were angry it was probably your adrenaline; warmer blood to heat up, expand, and loosen your muscles; along with your larynx's position. If you contract your Larynx/throat you get a higher pitch, and if you expand it you get a lower pitch. You can feel the difference if you put your hand on your throat. Your Laryngeal Prominence, Adam's Apple, will go up when you contract your Larynx, and down when you expand it. The further you expand the deeper your voice gets. However it is harder to speak if you are not accustomed to speaking in the expanded state. I suggest practice.

I would say yes, but that is my sadism talking.
If you can deal with the phone calls in the middle of the day and their constant defamation, then I would say yes. All one would need is to act a bit obsessively. When you spend time with them say things affirm their obsession, low self esteem; whatever their "yandre" may be, and you'll have a decent relationship.

You can, but she'll be jealous and think you are cheating on her. If she is "crazy" about you then you wont have to fear for your safety. If you come home one night and she is waiting in a dark room and asks you where you have been, all you have to do is be honest. Because she was probably watching you anyways; you can talk her down so long as you are calm and don't lie about anything physical. Things like "You are the only one for me.", "There's nothing to worry about.", or "I'm not going to leave you." are all words with no physical proof, and can calm her suspicions if need be. However things like "I wasn't with (Name) today.", "I didn't kiss her.", or "I'm not doing anything behind your back." are all things that can be proven or disproven through stalking. Avoid open areas when you choose to cheat on "Yandre", and you'll be fine. In my opinion you wouldn't need to cheat on a yandre because you could go all out on your sadism and they'll be with you. If they don't want to follow anything you tell them to do just ask "Why not don't you love me?"; you'll have to alter this sentence if you keep on encountering this problem but keep it to the same effect. Hug her a lot, over 20 seconds, to calm her down if she is upset, and give her affirmations to your "love".